Most geoengineering schemes fall into two categories — carbon-dioxide removers and sunlight blockers. These six ideas are being discussed now.

Brown Bird Design

Planting forests

THE IDEA Trees turn carbon dioxide into oxygen through photosynthesis. China, for instance, has planted about 60 billion trees in the last 30 years.

THE CATCH It takes time to grow trees, and a huge amount of land. They don’t all survive and thrive. Plus, areas of darker tree canopy absorb heat from the sun, reducing their overall cooling effect.

Brown Bird Design

Spraying sulfuric acid into the stratosphere

THE IDEA Evidence from volcanic eruptions shows that tiny sulfur particles are very good at scattering sunlight. Spraying a mist of these particles at a high altitude could cool the earth quickly and relatively cheaply (one study estimated a few billion dollars a year). The sulfur could be sprayed by aircraft, although some plans have suggested artillery, and one an 18-mile-long hose suspended by giant balloons.

THE CATCH Sulfur particles pose a danger to the ozone layer. Plus, like all plans to block sunlight, this would do nothing about the non-warming harms of carbon dioxide buildup such as ocean acidification. Finally, if you ever stopped the sprays without first removing a lot of atmospheric carbon dioxide, the planet would reheat at a much faster, and therefore more destructive, pace.

Brown Bird Design

Dumping iron into the ocean

THE IDEA Studies have shown that fertilizing the ocean with iron dust can trigger huge blooms of carbon dioxide-hungry phytoplankton.

THE CATCH It’s unclear how much of the plankton dies and falls to the seafloor, where its carbon gets trapped for centuries, and how much is eaten by other creatures, eventually cycling the carbon back into the atmosphere. Scientists also worry about plankton blooms throwing ecosystems out of balance and creating marine “dead zones” where bacteria that decompose dead plankton suck up all the available oxygen.

Brown Bird Design

Space mirrors

THE IDEA Either launch one huge (Greenland-size) mirror into orbit, or vast arrays of a trillion more-reasonably sized mirrors.

THE CATCH Even supporters say this scheme would cost at least $800 billion and maybe as much as $400 trillion. Also, no existing launch vehicles could handle that scale of payload. Positioning the mirrors correctly would also be a challenge, not to mention the task of cleaning and maintaining them or the complicated geopolitics of launching anything into space.

Brown Bird Design

Brightening the clouds

THE IDEA Clouds are masses of water vapor that have condensed into droplets around airborne particulates. Spraying saltwater at high pressure into the sky would create smaller-than-normal droplets, but more of them, which would mean more surface area to reflect sunlight.

THE CATCH A leading proponent of cloud brightening says that to brighten enough clouds for significant cooling, it would take about a thousand ships, likely wind-powered, operated by remote control, and equipped with (yet to be developed) sprayer systems.

Brown Bird Design

Industrial-scale carbon dioxide scrubbers

THE IDEA Several chemicals, such as sodium hydroxide, bind to carbon dioxide and can remove it from the air like a sponge. Chemical scrubbers are already used on factory smokestacks, and systems are being developed to grab carbon dioxide out of ambient air.

THE CATCH Building enough scrubbers to make a difference would be wildly expensive. For fighting global warming, some say the money would be better spent on keeping carbon dioxide out of the air in the first place.